Britain and Ireland have said that the passports used by nine people suspected of the killing of a senior Hamas figure in the United Arab Emirates are believed to be fake.
The nine passport holders are alleged members of an 11-person hit squad which Dubai police have said that they want to question over the murder of Mahmud al-Mabhouh in a Dubai hotel room on January 20.
A spokesman for Britain's foreign office said on Tuesday: "We are aware that the holders of six British passports have been named in this case.
"We believe the passports used were fraudulent and have begun our own investigation."
Earlier, the Irish foreign affairs department said it had checked the details of the three purported Irish passport holders provided by the Dubai police and found them also to be false.
"We have run the passport numbers and names through our system and there are no passports in those names or with those numbers," a department spokesman said.
"These purported passports are false. These are not genuine passports."
Dubai police officials released the passport details, photographs and surveillance footage of the 11 suspects on Monday.
The two remaining suspects were said to be the holders of a German and French passport.
Dubai prosecutors have issued an international arrest warrant against the 11 suspects, a statement released on Tuesday said.
Two Palestinians have been arrested over the killing, but the Hamas movement, which governs the Gaza Strip, has blamed Israeli agents for Mabhouh's death.
Dhafi Khalfan, Dubai's police chief, said on Monday that he had not ruled out "the involvement of Mossad [Israel's intelligence agency] or other parties in the assassination".
The group were believed to have rented a room across the corridor from al-Mabhouh around the time of the murder, spending just 24 hours in Dubai without using credit cards or local phone lines, the police chief said.
Khalfan showed a news conference airport surveillance video of the alleged assassination team arriving on separate flights before checking into separate hotels.
In the footage, which also included images from the hotel where al-Mabhouh was killed, the one woman among the group of suspects appears to be wearing a wig and at times wears a big hat and sunglasses.
Others were seen apparently posing as tourists, wearing tennis clothes and carrying rackets.
Authorities appear to have linked the group through the videos which show them entering and exiting the hotel, standing together in the hotel lobby and going in and out of the elevator on the floor where al-Mabhouh was staying.
Police said that the killers used an electronic device to enter Mabhouh's room and lay in wait for him.
Al Jazeera's Ayman Mohyeldin reports on the Dubai police's hunt for the Hamas figure's killer
"He was strangled after receiving maybe an electric shock," Khalfan said on Monday.
Al-Mabhouh entered the United Arab Emirates a day before his death using a passport that did not bear his family name, Khalfan said.
"Obviously, the gang knew that he was coming to Dubai, because they knew in advance and were able to come from Europe," he told the news conference.
Israeli authorities have not commented on the killing, but Israeli media have welcomed al-Mabhouh's death.
The Jerusalem Post
newspaper said that the murder was "another blow to the axis of evil".
Al-Mabhouh was born in the Gaza Strip, but had been living in Syria since 1989.
He is said to have engineered the capture of two Israeli soldiers during a Palestinian uprising in the 1980s and was imprisoned several times by Israeli forces.
Hamas has said he was an "important" member of Izz al-Din al-Qassam brigades, Hamas's military wing named after a Syrian religious leader who fought British colonial forces in Palestine in the 1930s.